Reaction kinetics of electrogenerated 9,10-bis(4-substituted-phenyl)anthracene cation radicals with methanol: A change in mechanism brought about by remote substituents

Abstract

The reactions of 9,10-bis(4-nitrophenyl)- and 9,10-bis(4-methoxyphenyl)anthracene cation radicals (DNA˙+ and DAA˙+) with methanol (MeOH) in acetonitrile were analyzed using a pulse electrolysis stopped-flow method. The reaction of DNA˙+ was found to proceed via the same rate law as the 9,10-diphenylanthracene cation radical (DPA˙+), i.e. −d[DNA˙+]/dt = k[DNA˙+][MeOH]2, with a reaction rate 6 times faster. In contrast, the rate law governing the reaction of DAA˙+ with MeOH was different from that of DPA˙+, and was found to be −d[DAA˙+]/dt = k[DAA˙+]2[MeOH]. Reflecting this difference in the rate laws, an acceleration of the reaction was observed in the decay of DAA˙+ relative to DPA˙+, though methoxy groups usually stabilize aromatic cation radicals. This unusual remote substituent effect, which involves a change in the reaction mechanism, is discussed in terms of stabilization of intermediates in the proposed reaction mechanisms.